1. Field of the Invention
The present invention relates to a polytrimethylene terephthalate resin. More particularly, the present invention is concerned with a polytrimethylene terephthalate resin comprised mainly of trimethylene terephthalate recurring units, which has the following characteristics: an intrinsic viscosity of from 0.8 to 4.0 dl/g; a molecular weight distribution (Mw/Mn) of from 2.0 to 2.7; a psychometric lightness L-value (L-1) of from 70 to 100 and a psychometric chroma b*-value (b*-1) of from −5 to 25; and a psychometric lightness L-value (L-2) of from 70 to 100 and a psychometric chroma b*-value (b*-2) of from −5 to 25 as measured after heating the polytrimethylene terephthalate resin at 180° C. for 24 hours in air. By using the polytrimethylene terephthalate resin of the present invention, a shaped article having high strength and excellent color can be stably produced on a commercial scale. Further, the present invention is also concerned with a method for stably producing the polytrimethylene terephthalate resin with high productivity on a commercial scale.
2. Prior Art
A polytrimethylene terephthalate resin (hereinafter, referred to as “PTT”) not only has characteristics similar to those of a nylon (e.g., soft feeling due to the low elasticity of the resin, excellent elastic recovery and good dyeability), but also has characteristics similar to those of a polyethylene terephthalate (hereinafter, referred to as “PET”) (e.g., wash and wear property, dimensional stability and discoloration resistance). Therefore, a PTT has been attracting attention as a raw material which can be used for producing carpets, clothes, shaped articles and the like.
For further expanding the application fields of a PTT, it has been desired to improve the strength and color of the fibers and shaped articles of a PTT.
For improving the strength of the fibers and shaped articles of a polymer, it is necessary to increase the polymerization degree of the polymer, and to narrow the molecular weight distribution of the polymer so as to reduce the amount of low molecular weight components in the polymer. Further, for improving the color of the fibers and shaped articles of a polymer, it is necessary not only to improve the whiteness of the polymer, but also to improve the heat resistance of the polymer so as to prevent the discoloration of the polymer, which is caused by the thermal history experienced by the polymer during the drying, melting and the like.
As a polymerization method for producing a PTT, a melt polymerization method is widely known. For example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 (corresponding to U.S. Pat. No. 5,340,909), WO98/23662, WO01/14450 and WO01/14451 disclose a method in which a melt polymerization is performed using a polymerization vessel equipped with a stirrer. The above-mentioned polymerization vessel has advantages in that it exhibits excellent volume efficiency and has a simple structure. Such a polymerization vessel can be used on a small scale for efficiently performing a polymerization to produce a polymer having a high polymerization degree. However, when the above-mentioned polymerization vessel is used for performing a polymerization on a commercial scale, the depth of the liquid reaction mixture in the polymerization vessel inevitably becomes deep, leading to a marked occurrence of heat decomposition of the polymer. Thus, a polymer having high polymerization degree cannot be produced on a commercial scale.
Various techniques have been proposed for producing a PTT having a high polymerization degree by melt polymerization. Examples of such techniques include a technique in which a lower alcohol diester of terephthalic acid and trimethylene glycol are subjected to a transesterification reaction and a polycondensation reaction in the presence of a titanium compound, wherein the molar ratio of the lower alcohol diester of terephthalic acid to trimethylene glycol is in the range of from 1/1.2 to 1/1.8 (Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-140992); a technique in which an organometal catalyst is used as a polycondensation catalyst, and an organic sulfonic acid or an aliphatic carboxylic acid is used as a catalyst auxiliary (U.S. Pat. No. 4,611,049); a technique in which a tin catalyst is used as a polycondensation catalyst (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 (corresponding to U.S. Pat. No. 5,340,909)); a technique in which a specific titanium catalyst is used as a polycondensation catalyst (Unexamined Japanese Patent Application Laid-Open Specification Nos. 2000-159875 and 2000-159876); a technique in which an antimony compound is used as a polycondensation catalyst (Chemical Fiber International Vol. 46, pp 263–264, 1996); a technique in which heat decomposition of a PTT is suppressed by using a hindered phenol-type stabilizer having a specific structure (Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-142097); and a technique in which the by-production of acrolein (formed by heating of a prepolymer and a polymer in air during the polymerization) is suppressed by blocking the terminals of the prepolymer and the polymer with a phosphorus-containing stabilizer and a hindered phenol-type stabilizer (WO98/23662 and WO99/11709). However, the above-mentioned techniques are disadvantageous in that the molecular weight of the obtained PPT is not satisfactorily high, in that a lowering of the molecular weight of the PTT occurs during the molding thereof, and/or in that a discoloration of the PTT occurs. Thus, by the above-mentioned techniques, a PTT having satisfactory properties cannot be obtained.
Further, a method is proposed in which, for the purpose of obtaining a high molecular weight PTT which exhibits excellent heat stability during the spinning of the PTT, a solid-phase polymerization of a PTT prepolymer having a relatively low molecular weight is performed, in which the PTT prepolymer has not suffered heat decomposition and has excellent color (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-311177, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. 2000-502392 and Korean Patent No. 1998-061618). However, the solid-phase polymerization proceeds while releasing trimethylene glycol (hereinafter, referred to as “TMG”) from the surface of pellets of the prepolymer during the polymerization reaction. Therefore, the polymerization degree varies depending on the size and shape of the pellets, and also varies depending on the position in the pellets. Therefore, the PTT obtained by this method is markedly non-uniform with respect to the polymerization degree. Further, in the solid-phase polymerization, the pellets of the solid prepolymer get rubbed with one another over a long period of time, thereby generating a polymer powder which becomes a loss. Furthermore, in the above-mentioned method, the solid-phase polymerization should be conducted after the production of the prepolymer by the melt polymerization and the like, and thus, the entire process for producing a PTT becomes complicated and costly. Still further, the presence of the polymer powder in the spinning process causes breakage or fuzzing of polymer fibers. For removing the polymer powder, an addition step therefor becomes necessary.
As a method for producing a PTT having a high polymerization degree only by melt polymerization, a technique has been proposed in which the polymerization is performed using a disc ring reactor or a cage type reactor (WO00/64962) or a disc and donut conductor (U.S. Pat. No. 5,599,900) to withdraw the TMG efficiently from the reaction system. However, each of the above-mentioned apparatuses is a horizontal agitation-type polymerizer which is equipped with a rotary driving part. Therefore, in the above-mentioned method, when a polymerization is performed under a high vacuum for obtaining a polymer having a high polymerization degree, it is impossible to seal the driving part completely. Thus, it is impossible to prevent the intrusion of a trace amount of oxygen into the polymer, and hence, a, discoloration of the polymer inevitably occurs. Especially, in the case of a PTT, such discoloration markedly occurs. When the driving part is sealed with a sealing liquid, it is likely that the sealing liquid gets mixed with the polymer, thereby lowering the quality of the resultant PTT. Further, even when the driving part of the apparatus is tightly sealed at the start of the operation thereof, the tightness of the sealing is lowered during the operation conducted for a long period of time. Thus, the above-mentioned method also has a serious problem with respect to the maintenance of the apparatuses.
On the other hand, a method for producing a resin (other than PTT) is known in which the polymerization apparatus used therein does not have a rotary driving part, and a polymerization is performed by allowing a prepolymer to fall from a perforated plate (free-fall polymerization method).
For example, a method is disclosed in which a polyester prepolymer is allowed to fall in the form of fibers in vacuo in an attempt to obtain a polyester having a desired molecular weight (U.S. Pat. No. 3,110,547). In this method, a polymerization reaction is performed in a one pass mode without recycling the polymer, because the recycling of a polymer which has already been allowed to fall in the form of fibers causes the lowering of the quality of the final polyester. However, the above-mentioned method has the following disadvantages. The polymer in the form of fibers are easily broken during the polymerization reaction, thereby causing a disadvantageously large variation in quality of the final condensation polymer products. In addition, a low molecular weight condensation polymer is scattered from the polymer fibers during the polymerization reaction to stain the lower surface of the perforated plate. Due to such staining of the lower surface of the perforated plate, it becomes difficult to cause the polymer to fall in the form of fibers, so that the polymer fibers contact with one another to cause breakage of the polymer fibers or the polymer fibers are combined together to form a thick fiber in which the reaction does not proceed efficiently.
In order to solve these problems, various methods have been proposed. Examples of such methods include a method in which a polyester or a polyamide is produced by allowing a prepolymer to fall along and in contact with the surface of a perforated guide or a wire guide, which is vertically arranged in a reaction vessel, so that the polymerization of the prepolymer is effected during the fall thereof (Examined Japanese Patent Application Publication No. Sho 48-8355 and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 53-17569); a method for continuously condensation-polymerizing bis-(β-hydroxyalkyl) terephthalate (which is an initial-stage condensation product of polyethylene terephthalate (PET)), in which bis-(β-hydroxyalkyl) terephthalate is allowed to fall along and in contact with wire guides in an atmosphere of inert gas, wherein the wire guides are hung vertically from the holes of a perforated plate, so that the polymerization of bis-(β-hydroxyalkyl) terephthalate is effected during the fall thereof (Examined Japanese Patent Application Publication No. Hei 4-58806); and a method for producing a melt-polycondensation polymer, such as a polyester, a polyamide and a polycarbonate, in which a melt-polycondensation prepolymer is caused to absorb an inert gas, and then, polymerized under reduced pressure (WO99/65970 which also discloses an apparatus used in the method).
However, each of the above patent documents only describes a method for producing a polyester, such as a PET, or nylon, and has no proposal or suggestion about the production of a PTT. As a result of the studies of the present inventors, it has been found that, when any of the above-mentioned methods are simply applied to the production of a PTT (that is, when the production of a PTT is conducted by the above-mentioned methods, using raw materials and conditions which are conventionally used in the production of a PTT), a foaming of a polymer vigorously occurs, thereby staining the lower surface of the perforated plate or the inner wall of the reaction vessel having the guides provided therein. A PTT is susceptible to heat decomposition, as compared to a PET. Therefore, the stain caused by the above-mentioned vigorous foaming of the polymer is easily decomposed. When the resultant decomposition products get mixed with the polymer, disadvantages are caused in that the quality of the polymer is lowered, in that the desired polymerization degree cannot be obtained, and in that the obtained PTT suffers discoloration. Further, the simple application of the above-mentioned methods to the production of PTT is accompanied by problems in that it is difficult to achieve a satisfactorily high polymerization degree, and in that the final PTT contains low molecular weight polymers, which result in a broad molecular weight distribution of the final polymer and are likely to lower the mechanical strength of an ultimate shaped article.
As mentioned above, the conventional methods for producing a PTT have the following problems:    (1) It is difficult to produce a PTT having a high polymerization degree only by melt polymerization (i.e., without solid-phase polymerization) on a commercial scale. When the production of a PTT is conducted by solid-phase polymerization, disadvantages are caused in that the molecular weight distribution of the obtained PTT becomes too broad, and in that the production process becomes complicated and costly (due to the loss caused by the formation of polymer powder).    (2) When it is attempted to produce a PTT having high polymerization degree by using a specific catalyst or stabilizer, the obtained polymer is likely to suffer heat decomposition and discoloration.
With respect to the free-fall method (in which a polymerization is performed by allowing a prepolymer to fall freely in the form of fibers from a perforated plate) and the guide-wetting fall method (in which a polymerization is performed by allowing a prepolymer to fall along and in contact with a guide), it is known that these methods can be used for producing polyamide and polyesters (such as a PET) other than a PTT. However, the application of the above-mentioned methods to the production of a PTT is not known, and a PTT having satisfactory properties cannot be obtained by a simple application of these methods to the production of a PTT which is different from the above-mentioned polyamides and other polyesters with respect to the melt viscosity, and resistance to heat decomposition, and volatilities of by-products.
For these reasons, there has been a demand for the development of a method which can be used for producing an excellent PTT having a high polymerization degree on a commercial scale, which PTT can be used for stably producing a shaped article having excellent strength and color.